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CAN WE LEARN FROM HEAT-STRESSED DAIRY COWS? SANTA GERTRUDIS Product ion

Randy L. Stanko, Ph.D., Texas A&M University-Kingsville R ecently, I attended an Animal Science Society Symposia lecture that summarized five years of research data on heat stress in

klers and fans were automatic and turned on when air tem- perature reached 70 degrees. All calves born were managed the same, received colostrum within four hours of birth and were weaned at 49 days of age. Research Findings Cooled calves were heavier at birth (by 12.5 pounds) as compared to heat-stressed calves. The stillborn rate was 4.1 percent and 0 percent for the heat stress and cooled calves, respectively. Body weight, growth rate, fertility and milk pro- duction in the first lactation were followed in 72 heifer calves born to these dairy cows (heat stress, n=34; cooled, n=38). Cooled calves were heavier and taller up to 1 year of age but had similar total weight gain as compared to heat stressed heifers. No effect of treatment was observed on age at first artificial insemination (AI) or age at first parturition. Heifers born to heat-stressed cows had a greater number of AI services per pregnancy confirmed at 30 days, but treat- ment did not affect number of AI services per pregnancy confirmed at 50 days. More heifers from cooled dairy cows reached first lactation compared to heifers born to heat- stressed cows, 85.4 percent vs. 65.9 percent, respectively. In addition, heifers from heat-stressed dams produced less milk up to 35 weeks of the first lactation as compared to heifers from cooled cows. This is very interesting data and should make all fall-calv- ing unit mangers think about August/September heat and humidity. How much shade do I have in my pastures? More- over, how are those heavy-bred cows handling the heat load? We are learning more and more about maternal stressors and fetal programming, both human and livestock. There is no doubt that maternal stress during late gestation can affect the offspring, both early and later in life. The exact physi- ological mechanism remains unknown, but the last trimester appears to be the most susceptible for heat stress to affect calves in utero. As I have stated previously, dairy cows and beef cows are different beasts. I am not suggesting running extension cords and fans into each pasture, nor calling a plumber to run water lines and sprinkler set ups in each pasture. As we are coming out of the “dog days” of summer, I simply suggest you think about this dairy cow data (do more reading if you like) and your heavy-bred, fall-calving cows’ shade availabil- ity. I am not aware of any similar studies involving beef cows. If nothing else, this data serves as a good reminder to why we love our American-breed type cattle.

dairy cattle. G.E. Dahl and colleagues at the University of Florida Dairy Unit in Hague, Fla., did the research studies (2016, J. Dairy Sci., 99:8443-8450). The information presented made me think about our southern states beef producers with a fall calving season. I fully realize that dairy cows are pheno-

typically different from beef cows, and American-breed cattle are phenotypically different as compared to Bos taurus cattle breeds. Nonetheless, they are all still cattle. Thus, I want to share with you this very interesting data and possibly suggest that you evaluate your summer shade availability. Background The study used dairy cows of similar lactation production to determine if heat stress in utero has carryover effects on the heifers produced. Previously, this group of researchers have shown that calves born to heat-stressed (during late gestation) dairy cows have lower birth and weaning weights and compromised passive immune transfer from colostrum compared to those born to cooled cows. This compromised immune transfer occurred regardless of colostrum source (heat stressed, cooled or mixed). In this study, dairy cows were dried off 46 days before expected calving and randomly assigned to either heat-stress or cooled treatment. The heat stress or cooling always occurred from Aug. 20 to Sept. 17, over a five-year study. Recall the research occurred at Hague, Fla., (north central Florida). The heat-stressed dairy cow treatment cows had access only to shade. The cooled dairy cows had access to sprinklers, fans and shade. Sprin- How much shade do I have in my pastures? Moreover, how are those heavy-bred cows handling the heat load? We are learning more and more about maternal stressors and fetal programming, both human and livestock. There is no doubt that maternal stress during late gestation can affect the offspring, both early and later in life.

George West, Texas (361) 566-2244 lacampanaranch.com campana@granderiver.net

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